The term “flat steel product” is understood here to mean steel sheets or steel strips produced by a rolling process and sheet bars and the like that have been divided therefrom.
Requirements in the automobile construction sector are increasing ever further in respect of passenger safety in the event of a crash. At the same time, lightweight construction is an important prerequisite for compliance with the legally stipulated CO2 limits and for the minimization of the energy input required to drive the vehicle. There are likewise ever higher demands in terms of comfort on the part of users of vehicles, which leads to an increased level of electronic components in the vehicle and an increasing vehicle weight as a result. In order to simultaneously fulfill these opposing requirements, the focus has long been on lightweight design in the manufacture of bodywork structures for automobile construction.
For automobile components of relevance in respect of crash safety, components which have become established here are especially those which are produced by hot forming, followed by hardening, of flat steel products consisting of a manganese-boron steel. This manufacturing method, which is also referred to in the field as press-hardening, can produce components which, in spite of optimally low wall thicknesses and associated minimized weight, can be used at locations in a vehicle body that are particularly sensitive in respect of their performance in the event of a crash.
It should be pointed out that the present invention is not restricted solely to automobile construction, but can generally be employed in the industrial sector. In all fields, but specifically in the automotive sector, different demands are made locally on the components with regard to their material properties. Examples of material properties include strength and ductility. “Locally” means that different demands are made on a first section of the component than on a second section. Since these demands can even be opposing, a solution having singular construction materials, i.e. having the same material properties throughout, is at best a compromise of all the individual demands made. It is therefore not only within the context of efforts to achieve lightweight construction in automobile construction that tailored materials are increasingly being used, these enabling optimization of components, for example in terms of their weight via reduction in the sheet thickness, by virtue of locally different material properties. However, the achievement of lightweight construction beyond this level also additionally requires the fulfilment of material properties, some of them contrary, within individual component sections. This means that, for consistent pursuit of lightweight construction, not only are locally different material properties in the sheet plane desirable, i.e. in longitudinal direction (direction of strip length) and in transverse direction (direction of strip width), but also at right angles thereto (direction of sheet thickness).
Longitudinal direction will also be referred to hereinafter as X direction, transverse direction will also be referred to as Y direction, and sheet thickness direction will also be referred to as Z direction.
The prior art discloses, in the mass production of components, the enablement of the establishment of locally different material properties within the sheet plane via methods such as tailored blanks (cold or hot forming) or tailored tempering (hot forming). Function-optimized modelling of material properties in the sheet thickness direction (Z direction) can be achieved through the use of multilayer flat steel products.
A multilayer flat steel product consisting of a multitude of mutually bonded steel alloy layers with at least one core layer is described, for example, by DE 10 2007 022 453 A1. The core layer here is to consist of a soft steel alloy of good formability, whereas the outer layers of the flat steel product are to consist of an ultrahigh-strength steel alloy.
In the press release under the link www.thyssenkrupp-steel-europe.com/en/presse/pressrelease.jsp?cid=2778030 dated 27 May 2014, the applicant publicized a new product named TriBond®—“three layer steel composite for hot forming”, which is a multilayer flat steel product having a core layer having a relatively high carbon content and outer layers having a relatively low carbon content. The flat steel product is intended for production of a component for a motor vehicle body, said component being hot-formed and hardened.